1. Field of the Invention
The present invention relates generally to roller cone rock bits that operate in “drilling mud.” More specifically, the invention relates to roller cone rock bits having nozzle arrangements configured to provide enhanced cutting structure cleaning while offering the potential to reduce cone erosion.
2. Background of the Invention
Rotary cone rock bits are used for drilling boreholes in various earth formations. Roller cone bits are conventionally manufactured using a segmented arc construction method. This method utilizes three 120° arcuate leg segments, each comprising one-third of the drill bit body and an individual leg portion with a bearing journal thereon. The three arcuate sections are usually forged and then machined to form the bearing surfaces on the bearing journals and the mating surfaces along each edge where the three sections are welded together. Prior to joining the three arcuate sections to form a bit body, the cutter assembly with bearings and retention means are mounted on the inwardly projecting bearing journals due to space limitations and associated difficulties of mounting the cutters after the three arcuate sections have been welded together. After the bearing assemblies and cutters are mounted on the machined bearing journals, the three leg segments are then placed in a welding jig and welded to each other to form the bit body. After welding has occurred a tapered thread is machined on the upper end of the bit, commonly referred to as the “pin” end, and the bit is ready for use.
To aid in the removal of drilling cuttings from the bottom of the borehole, mud or other fluids are introduced to the bottom of the borehole through nozzles or jet assemblies positioned in bores formed in the bit body. The fluid or mud pumped through the bit body also serves to remove heat from the rock bit. In general, as the efficiency of the cutting removal is increased, the cutting efficiency and associated rate of penetration (ROP) are also increased.
To efficiently remove cuttings from the borehole, the fluid or mud must carry the cuttings radially outward on the borehole bottom and then upward through the annulus (i.e., the annular space formed between the bit body and drill string, and the borehole). A fluid flow that will carry the cuttings radially outward from the center of the borehole and up the annulus may be created by introducing a downward fluid flow on the cutting structure of the bit. As the fluid flows past the cutting structure, the fluid impacts the borehole bottom and spreads radially outward to the annulus.
The amount of energy available at the bit is generally dictated by factors external to the bit such as the drilling rigs' available hydraulic energy, drill pipe type, bottom hole assembly (BHA) configuration and drill depth. However, once the available energy for the rock bit is determined, properly configuring the hydraulics of the bit for the specific application can significantly affect the rate of penetration (ROP) of the bit in the formation.
When drilling softer formations and plastic formations, there is a strong tendency for cuttings to adhere to the teeth or inserts of bits. The adhesion of formation to teeth or inserts is commonly referred to as “bit balling”. As is known in the art, bit balling describes the packing of formation between the cones and bit body, or between the cutting elements on the bit, during drilling. When bit balling occurs, the cutting elements may be sufficiently packed off such that they are limited in their ability to effectively penetrate into the formation, thereby tending to slow the ROP of the drill bit. Thus, cuttings must be removed efficiently during drilling to maintain reasonable penetration rates.
In harder clays and shales, cuttings can become impacted or “balled up” between the teeth or inserts of the cutting structures. When formation sticks to cones or is impacted between cutting elements it limits the ability of the cutting element to penetrate the formation. Also, formation packed against the cone-shell constricts the flow channels needed to carry cuttings away. This promotes premature bit wear. In either instance, having sufficient drilling fluid directed toward the cones can help to clean the inserts and cones, allowing them to penetrate to a greater depth, thus maintaining the ROP of the bit. Furthermore, bit durability may be improved, even as the inserts begin to wear down since the cleaned inserts will continue to penetrate the formation even in their reduced state.
To combat bit balling and to allow for larger cones, the cutting elements on adjacent rolling cones are often arranged to intermesh. As a cutting element of a first cone intermeshes between two rows of cutting elements of a second cone, it dislodges formation packed between the two rows of cutting elements. Having the cutting elements intermesh also allows the diameter of the cones to be larger, providing for a larger bearing surface which results in a more durable cone.
To further facilitate cuttings removal, jets or nozzles are positioned on the bit to wash cuttings from the cutting structure and/or borehole bottom. Conventional nozzle arrangements include the placement of a nozzle between each of the cones proximal the outer periphery of the bit. In many applications, a jet may also be located at the center of the bit to channel fluid flow directly to the borehole bottom. For example, U.S. Pat. No. 5,853,055 describes an extended center jet on a bit to reduce diffusion and provide a substantially uninterrupted fluid flow that strikes the borehole bottom with maximum impact energy. U.S. Pat. No. 6,290,006 describes special nozzles that can be used to provide collimated streams of high velocity drilling fluid that strike different areas of the bottomhole with maximum energy for enhanced bottomhole cleaning. An alternative nozzle arrangement involves a jet positioned between each of the cones near the periphery of the bit body. U.S. Pat. No. 4,611,673 proposes orienting nozzles toward one of the adjacent cones so that the jet of fluid exiting the nozzle will strike the cutting elements on the cone. However, these arrangements may not be desirable in applications where bit balling is a concern because they do not provide cleaning along interior rows of cutting elements on the cones, and further, a concentrated flow of drilling fluid directed toward and impacting a cone shell may result in excessive erosion of the cones and premature loss of cutting elements from the cones.
To avoid balling problems, bits have been designed with additional nozzles positioned over each of the cones which direct a jet stream of fluid directly on top of the cones in the dome region. One problem with these designs is that the impact of fluid directly on top of the cones often results in severe erosion of the cone shell and a premature loss of cutting elements from the cones. The placement of these nozzles has been restricted in the prior art to nozzle bores where the nozzles are installed through the pin due to limited access to the dome region from the journal side of the forging to drill the required nozzle bores.
In the prior art, a cast bit body was proposed that allowed for the machining and installation of nozzles before the leg portions were attached to the body. Unfortunately, the method requires a cast body formed separately from the leg portions. In practice, this type of cast body method is not typically used, and is generally limited to the manufacture of extremely large bits. Further, these designs may require insertion of the nozzles in the bit body prior to assembly of the legs on the bit. In practice, nozzles are typically inserted in the field after assembly of the bit based on rig specifications and drilling requirements particular to a specific application.
Numerous efforts have been made by drill bit designers to solve the problem of bit balling yet the problem persists. Accordingly, there is a need in the art for bits having an improved nozzle configuration that provides sufficient cleaning for cutting elements along the inner rows and outer rows of the cones to minimize bit balling. Such bits with improved nozzle configurations would be particularly well received if they reduced impingement of the cone shell with drilling fluid, thereby offering the potential to reduce the likelihood of cone shell erosion and associated premature loss of cutting elements.